I am not so sure that pre-child happiness is a good predictor. Going to the theater together might have been great, but those days are over.
Great sex may be a negative predictor. The less frequent and thrilling the sex, the easier it is to give it up and focus on the kid.
So what are the predictors?
1. Lack of other common interests, beyond the child. That makes it easier for the child to be the new center of attention.
2. Religion, and the belief that child-rearing is essential to God's plan, and not just in terms of agnotheist Bayesian expected value.
3. Ability to talk oneself out of building resentments, which are inevitable.
4. Clearly defined division of labor within the household.
5. Similar taste in television programs and/or childrens' movies.
6. Resources to hire outside help, or nearby friendly and grandchild-loving parents.
I suspect #2 and #3 are most important on this list, maybe #6 too.